It takes a while, granted, but one can propagate Aeschynanthus from individual leaves. My A. speciosus was originally from individual leaves we propagated long ago at work, and I have at least one A. radicans leaf that's rooted and producing new growth now too, so I'm thinking maybe it's a general characteristic of the genus. This particular leaf didn't even take that long (about 3 1/2 months), though as you can see, the others in this pot are taking longer, and there's another pot of leaves where nothing's happened yet, so it's just one leaf out of I think eight. But still.
If you go looking for information about how to propagate Aeschynanthus, almost nobody says anything about this: virtually everyone says stem tip cuttings, and nothing more. (A few sites also suggest seeds or layering.) While it's probably true that stem cuttings will start faster and produce a fuller looking plant sooner than leaves, it's still neat that leaves are an option.
I ran into the website of the North Star African Violet Council Twin Cities (surely "Twin Cities' African Violet Council" wasn't taken already?) while looking around the internet for more on this, and they say that Streptocarpus, Chirita, Saintpaulia, and Petrocosmeas are typically propagated from single leaves. Nautilocalyx are also supposed to be easy to root from leaves, which I'll have to try at some point with my N. forgetii.
The genera of gesneriads that don't propagate well from leaves include Episcia (though I've done it, last summer), Columnea, Nematanthus, some Sinningias, and Codonanthe, according to Jon Dixon, the source being quoted by the NSAVCTC. (I'm condensing the list, so if you're interested, head over to the site and read their post.)
Weirdly, I've had better luck at home with Aeschynanthus and Episcia leaf propagation than I've had with Saintpaulia, even though African violets are supposed to be much easier. I don't have any trouble getting African violet leaves to root, but they stop there -- they don't go on to grow new leaves. When we tried doing this at the ex-job, on the other hand, they propagated fine. Sometimes they even propagated when we didn't intend them to. So obviously I'm missing something.
But anyway. There isn't necessarily a lot of practical application for this method, but now you know it can happen.